Global Warming

Seasonal forecasting 85

Weather forecasting and chaoscontinued

Figure 5.8Analyses and forecasts of the surface pressure in millibars (1 mbar=100 Pa=1hPa) for the

storm which passed over southern England on 15/16 October 1987. The fully developed storm is shown in

(b) about 4 hours before it reached the London area; (a) is the situation twenty-four hours earlier from

which the storm developed. (c) and (d) are twenty-four-hour forecasts with the Meteorological Office

operational fine-mesh model, (c) being the one available at the time and (d) being one produced after the

event using more complete data and better assimilation procedures.

given situation is to employensembleforecasting in which an ensemble
of forecasts is run from a cluster of initial states that are generated by
adding to an initial state small perturbations that are within the range of
observational or analysis errors. The forecasts provided from the means
of such ensembles show significant improvement compared with indi-
vidual forecasts. Further, ensemble forecasts where the spread amongst
the ensemble is low possess more skill that those where the spread in the
ensemble is comparatively high.

Seasonal forecasting

So far short-term forecasts of detailed weather have been considered.
After twenty days or so they run out of skill. What about further into the